Citing a study conducted in Alaska by the USDA’s Agricultural Research Service, Lee Van Wychen said, “The research showed significant numbers of nonnative weed species were hitchhiking across the state in the same containers as ornamental flowers and shrubs.” The number of nonnative species in Alaska has surged over the past few years.

Container-grown vegetables, herbs, perennials, trees and shrubs were included in the study. Plants came from within Alaska as well as from Canada and the western United States. Researchers incubated soil from the containers and found 54 weeds or invasive plants sprouted. Three species were native to Alaska.

The researchers note these variables in the number of seeds that sprouted from any given soil sample:

Soil from balled or burlap-covered trees and shrubs held more weed seeds than vegetable, herb and perennial containers.

More weeds sprouted from soil-based mixes and mineral soil than from sterile, soilless potting mixes like sphagnum peat moss.

The grower or vendor made a difference. Some had superior weed control practices and sold plants with few if any weeds.

Van Wychen noted that gardeners can help stop the spread of weeds by carefully watching for weeds sprouting around newly bought and transplanted plants, and pulling them immediately.